1. Field of the Invention
The present invention relates to solar cells, particularly a solar cell with its back surface having a paste electrode of Al thereon, and relates to methods and apparatuses for manufacturing the solar cell.
2. Description of the Background Art
FIG. 9 shows a structure of a solar cell with its back surface provided with a paste electrode made of Al. The structure of the solar cell is hereinafter described in connection with a manufacturing process shown in FIG. 10. For a crystalline silicon-based cell, after a p-type silicon substrate 1 is etched, an n-type diffusion layer 2 is deposited on one side of the substrate that serves as a photo-receiving plane and an anti-reflection film 3 is formed thereon in order to decrease the surface reflectance. On the other side of the substrate that is opposite to the photo-receiving plane (the above-mentioned other side is herein referred to as “back surface” as appropriate), a paste of Al is screen-printed, dried at approximately 150° C. and thereafter fired in the air at approximately 700° C. to form a paste electrode 4 of Al. Moreover, a silver paste is screen-printed on some parts of the back surface and the photo-receiving plane according to a pattern, dried and thereafter fired in an oxidizing atmosphere at a high temperature to form paste electrodes 5 and 6 of silver. The resultant device is immersed in a flux, silver paste electrodes 5 and 6 are then solder-coated, and the device is rinsed and dried to produce the solar cell. The solar cell generally has a size of, for example, 10 cm, 12.5 cm or 15.5 cm per side.
In the screen-printing process, a cell to be subjected to printing is fixed on a stage, and a screen mask is lowered to adjust the distance between the cell and the screen mask to an appropriate one. The Al paste is supplied onto the screen mask and a squeegee is moved while pressurizing the paste so as to transfer the Al paste onto the cell through the screen mask.
FIG. 11A is a plan view of a p-type silicon substrate 11 showing an Al paste electrode 14 formed on the back surface of the substrate. FIG. 11B shows a cross section of the substrate along line a-a in FIG. 11A. The thickness of the Al paste which has been dried is 45 to 55 μm, the average thickness being approximately 50 μm.
In order to manufacture solar cells excellent in long-term reliability at low cost, there has recently emerged a need for decrease of the amount of the Al paste used in the process that constitutes a considerable part of a solar cell. In addition, as it is known that decrease of the thickness of the Al paste electrode is effective for lessening any warp of the solar cell and, in this sense, it is urgently required to decrease the amount of the Al paste.
The decrease of the amount of the Al paste and the decrease of the thickness after drying to 40 μm or less for example, are unsatisfactory for the following reason. Referring to FIG. 12, after the Al paste is fired, ball-shaped Al particles 19 of a diameter ranging from several tens of μm to several hundred μm are generated on the outer edge of the electrode. This trouble (called “ball-up”) causes a problem that the cell cannot be commercialized due to the defect in appearance.